The expression of the gene coding for cholecystokinin (CCK) was studied pre- and post-natally in rat brain with a preproCCK specific cDNA probe to quantify mRNA. In addition, a c-terminal specific radioimmunoassay was used to measure CCK immunoreactive peptides (CCK-IR). A single band of mRNA corresponding to about 800 bases was observed after agarose gel electrophoresis of poly(A)+ RNA isolated from brain and Northern blot hybridization with the labeled preproCCK cDNA probe. Braine preproCCK mRNA was detectable by embryonic day 14 and progressively increased to reach maximum levels two weeks after birth. It tended to decrease slightly in adult rats. CCK-IR was undetectable at embryonic day 14. At embryonic day 20, detectable levels were measured but marked development was seen only after birth with a rapid increase between postnatal days 7 and 21. By three weeks, the concentration reached approximately 90% of the adult levels. Specific mRNA synthesis precedes pronounced appearance of the synthetic and storage machinery needed for neurotransmitter peptide function. The pattern of the ratio of CCK mRNA:CCK-IR from embryonic day 21 to adulthood suggests that synthesis or storage of the peptide alter during development.